1. Field of the Invention
The invention relates to a particle beam apparatus and a device for energy-corrected deflection of a particle beam of charged particles, and a method for energy-corrected deflection of a particle beam.
2. Description of the Related Art
Devices for deflection of particle beams of charged particles are utilized for particle beam apparatuses like, for example, electron microscopes, apparatuses of electron- or ion-lithography and for display apparatuses. For those applications, a particle beam of charged particles is to be deflected from an incoming direction to variable outgoing directions in order to reach different positions on a target surface.
The deflection of particle beams from an incoming direction to an outgoing direction generally results from applying electric or magnetic fields which apply lateral forces to the particle beam. The electric or magnetic fields are generated by deflectors that include electric or magnetic multipoles like, for example dipoles or quadrupoles. Thereby, the deflection angle between the incoming direction and the outgoing direction αm caused by a homogeneous magnetic field of strength B perpendicular to the particle beam, is given byαm=km/SQRT (W),   (1)whereby SQRT(W) is the square root of the kinetic particle energy W of the charged particles and km a constant, depending on particle mass, particle charge, the magnetic field strength and the angle between the magnetic field and the particle beam direction. Equation 1 is particularly valid in the case that the deflection angle is small compared to the angle between the particle beam and the magnetic field.
The deflection angle between the incoming direction and the outgoing direction αe, which is caused by a homogeneous electric field of strengths E perpendicular to the particle beam, is given byαe=ke/W,whereby ke is a constant, depending on particle mass, particle charge, the electric field strengths and the angle between the electric field and particle beam direction. Again, equation (2) is particularly valid in the case that the deflection angle is small compared to the angle between the particle beam and the electric field.
Because of the deflection angle's dependency on the particle energy W, only particles with a predetermined energy value become deflected by the predetermined deflection angle. Particles with an energy higher than the predetermined energy value are deflected less, and particles with a lower energy are deflected more. Since, in practice, the charged particles of a particle beam have a certain energy spread, the deflection angle's dependency on the energy of the charged particles results in an energy-depending dispersing of the particle beam when deflected. An energy-depending dispersing of a particle beam, also called chromatic aberration or “Farbfehler”, in often undesired since it may limit the spatial resolution of the particle beam apparatuses, particularly electron microscopes, apparatuses for electron- or ion-lithography and display apparatuses.
A device in which a particle beam of charged particles is deflected by a predetermined deflection angle in an energy-corrected way is disclosed in patent application U.S. Pat. No. 4,362,945. Therein, electrical fields and magnetic fields are superimposed perpendicular with respect to each other, whereby the forces of the electric fields on the charged particles counteract the forces of the magnetic fields on the charged particles. In addition, the forces of the magnetic field are dimensioned in such a way that the predetermined magnetic deflection angle αm is twice as large as the predetermined electric deflection angle αe, whereby the energy-depending deviations from the predetermined deflection angle compensate each other. Thus, in the case that the energy-depending deviation is compensated, the deflection angle α is:α=αm−αe=½αm  (3)
The compensation of the energy-depending dispersing of the particle beam results from equation (1) and (2), from which follows:δαm/δw=−½αmw  (1)
andδαe/δw=−αe/W   (5)
Equations (4) and (5) indicate that for the same deflection angles αm and αe, the energy-depending dispersing of the particle beam due to an electric field is essentially twice as large as the one due to a magnetic field.
It is further to be said that the compensation of the energy-depending deviations is improved, the closer the particle beam deflection positions of the electric field and of the magnetic field are located to each other. The particle beam deflection position of a deflector is given by the intersection point of the optical axis of the incoming particle beam with the axis of the particle beam leaving the respective field.
The complexity for building particle beam apparatuses with energy correcting deflectors that superimpose electric and magnetic fields is considerable, since the fields for such deflectors must be higher than for deflectors without energy correction. The reason is that the fields must not only deflect the particle beam, but also compensate each other. The multipole elements for the generation of such fields thus are required to be larger and more complex; the generation of stronger magnetic fields requires, for example, larger coils or higher coil currents within the particle beam region, which in turn may require additional cooling.
Further, the deflectors require a larger aperture to make sure that also particle beams at large deflection angles can be guided through the deflector. A large aperture for the energy-corrected deflector, however, requires large deflection plate distances and large coil distances. In order to still provide the required electric and magnetic fields in the area of the particle beam, the electric potential at the deflection plates and the currents in the coils have to be scaled to correspondingly higher values. This leads to a further enlargement or the coils, the voltages and/or the coil currents of the deflector, which in turn further increases the volume and further limits the possible usage.